The object of the invention relates to wideband dual-channel antennas for applications in which the bulk parameter is predominant. It targets in particular the antennas whose working frequencies are situated in the metric and decimetric wavebands, commonly called VHF (Very High Frequency) and UHF (Ultra High Frequency). The values for these frequency bands are, for example, for the VHF band, designated low band: 30-88 MHz, and, for the UHF band, designated high band: 225-520 MHz.
The expression “dual-channel antenna” should be understood to mean an antenna made from at least two radiant elements which are fed separately by means of two channels. These radiant elements may be activated simultaneously in certain cases in both frequency bands or else alternately.
It is known practice to produce a double antenna with a unipolar-type radiant element designed for the low frequencies, topped by a dipole-type radiant element designed for the high frequencies and fed through the unipolar-type radiant element. Most of the antennas of this type operate in a frequency band which is insufficient for certain applications.
The patent EP 0 851 532 (FIG. 1) describes a double antenna according to the prior art, in particular for vehicles, characterized in that it comprises a ground plane pierced by an orifice, with, on one side of the ground plane, the space outside the vehicle and, on the other side, a protected space; a coaxial cable 2 passes through the orifice, with a first end in the space outside the vehicle and a second end in the protected space, the cable forming turns 22 between the orifice and its second end; a link between the outer conductor of the cable and the ground plane at the second end; a radiant element of dipole type 1 connected to be fed, to the first end of the cable; a transformer consisting of a magnetic core 5 and the turns 22 whose secondary comprises the outer conductor of the turns, an impedance 6 with a value equal to the characteristic impedance of the cable, two feed cables 8a, 8b. The core of the cable 8a is connected to the point P situated on the outer braid of the portion of coaxial cable forming the turns 22 and intended to convey the RF signals of the VHF band or low band for the application. The coaxial 8b, however, has its core linked to that of the coaxial cable 2 through the duplexer 9 and conveys the RF signals of the UHF band designated as high band. The low band is fed via the braid of the coaxial cable 2. The antenna elements consist of monopole and dipole.
One of the drawbacks with such an antenna is that it is not suited, because of its size, to portable-type applications, for which the bulk/efficiency pairing is a crucial factor.
The patent U.S. 5,311,201 describes an antenna capable of operating in the AM/FM radio broadcast bands and also in the higher frequency band reserved for mobile radio. This antenna with two ports for vehicles intended for the reception only of the AM/FM broadcast radio and the transmission/reception of the UHF radiotelephone band (GSM) has the drawback of providing only reception on one of the channels.